Oscillations of weakly viscous conducting liquid drops in a strong magnetic field

TL;DR

This paper analyzes how a strong magnetic field influences the oscillations of weakly viscous conducting liquid drops, revealing that magnetic fields constrain flow, alter frequencies, and affect damping, with implications for material property measurements.

Contribution

It provides an asymptotic analysis showing magnetic fields do not change shape modes but significantly influence oscillation frequencies and damping in conducting liquid drops.

Findings

Magnetic field constrains flow, leading to 2D oscillations aligned with the field.

Oscillation frequencies decrease due to increased effective inertia from the magnetic field.

Magnetic damping decreases with stronger fields, affecting viscous damping significance.

Abstract

We analyse small-amplitude oscillations of a weakly viscous electrically conducting liquid drop in a strong uniform DC magnetic field. An asymptotic solution is obtained showing that the magnetic field does not affect the shape eigenmodes, which remain the spherical harmonics as in the non-magnetic case. Strong magnetic field, however, constrains the liquid flow associated with the oscillations and, thus, reduces the oscillation frequencies by increasing effective inertia of the liquid. In such a field, liquid oscillates in a two-dimensional (2D) way as solid columns aligned with the field. Two types of oscillations are possible: longitudinal and transversal to the field. Such oscillations are weakly damped by a strong magnetic field - the stronger the field, the weaker the damping, except for the axisymmetric transversal and inherently 2D modes. The former are overdamped because of being incompatible with the incompressibility constraint, whereas the latter are not affected at all because of being naturally invariant along the field. Since the magnetic damping for all other modes decreases inversely with the square of the field strength, viscous damping may become important in a sufficiently strong magnetic field. The viscous damping is found analytically by a simple energy dissipation approach which is shown for the longitudinal modes to be equivalent to a much more complicated eigenvalue perturbation technique. This study provides a theoretical basis for the development of new measurement methods of surface tension, viscosity and the electrical conductivity of liquid metals using the oscillating drop technique in a strong superimposed DC magnetic field.